Add rustfmt configuration and enforce in CI

This prevents any dummies like me from accidentally having a
different rustfmt configuration picked up from a parent directory,
since rustfmt will scan all the way up to the root directory to
find one. It will also ensure that CI builds will break if
misformatted code is uploaded.

To fail early and avoid unnecessary cycles of bad formatting being
committed and then fixed, it is a good idea to add a
`.git/hooks/pre-commit` file locally that checks staged files for
formatting errors:

```sh
git diff --name-only --cached -z '*.rs' |xargs -0 rustfmt --check || exit 1
```

Author: csnover

.github/workflows/main.yml

@@ -18,6 +18,7 @@ jobs:
         features:
           - name: all features
             value: --all-features
+            check_formatting: True
           - name: no_std
             value: --no-default-features --manifest-path binrw/Cargo.toml
     steps:
@@ -29,7 +30,15 @@ jobs:
           profile: minimal
           toolchain: stable
           override: true
-          components: clippy
+          components: clippy, rustfmt
+      - name: Check formatting
+        # There is no reason to check formatting more than once since it is
+        # a syntax check that does not change depending upon compiler features
+        if: matrix.features.check_formatting
+        uses: actions-rs/cargo@v1
+        with:
+          command: fmt
+          args: -- --check
       - name: Run clippy
         uses: actions-rs/cargo@v1
         with:

rustfmt.toml

@@ -0,0 +1,3 @@
+# rustfmt will search all the way up the directory tree to find a config
+# file. This file stops rustfmt from going beyond the top of the repo.
+# It is intentionally empty.